Petunias are hardy annual flowers available in a rainbow of colors. Grandiflora petunias grow 8 to 12 inches tall with wavy-edged flowers. Multiflora petunias have small, single blossoms on compact stems. Milliflora petunias produce tiny flowers that are only 1 inch across. Cascade petunias produce spreading 18-inch long stems that grow well in hanging baskets. The best varieties of petunias are available by seeds. In cold climates, plant the petunias seeds indoors to protect the seeds from the weather and get a head start on the growing season.
Wash a shallow seed tray with soapy water. Rinse the container with one part bleach and nine parts water. Starting petunia seeds indoors is done four to six weeks before the last spring frost date.
Fill the seed tray with a seed starter soil mixture. Create you own starter soil by mixing together four parts peat moss, two parts perlite and two parts vermiculite. This creates a lightweight, good-draining soil mixture where the seedlings can develop strong roots.
Spread the petunia seeds across the soil in the seed tray. Do not cover the seeds with soil since they need exposure to light to germinate. Spray the seed tray with a fine mist of water until the soil is wet.
Cover the seed tray with plastic wrap to form a miniature greenhouse. This produces high humidity and prevents the soil from drying out. Place the seed tray in a warm area located in bright indirect light. Under ideal conditions, the seeds will sprout in three to four days.
Remove the plastic wrap when most of the seeds have emerged. Allow the soil to dry out slightly between waterings. Spray with room temperature water as the seedlings grow.
Transplant the petunia seedlings to small, individual pots when the plants have grown two leaves in about 14 to 21 days. Use the seed starter soil mixture in the small plant pots.